E. coli O157:H7 is a virulent and common foodborne pathogen. Most outbreaks, and many sporadic cases (38,42; see the appended Citations), have been attributed to food of bovine origin. Most E. coli O157:H7 infections are sporadic, but this organism can cause massive epidemics by contamination of ground beef (19) and water (69). E. coli O157:H7 is transmissible from person to person, but the disappearance of the strain which caused the massive 1993 outbreak in Washington State soon after recall of the incriminated vehicle demonstrates that ingestion of contaminated beef, and not person to person spread, is the chief source of human infection.
E. coli O157:H7 organism elaborates Shiga-like toxins (SLT) I and/or II. SLT I and II inhibit protein synthesis by disrupting a glycosidic bond at a specific adenine (A4324) in 28S rRNA of the 60S ribosomal subunit. SLT-producing E. coli (SLTEC) are ubiquitous in food (62) and animals (47). The vast majority are probably not human pathogens.
Current data suggest that E. coli O157:H7 is the most common and medically significant SLTEC. Only one outbreak of bloody diarrhea caused by SLTEC other than E. coli O157:H7 has ever been reported (11). Additionally, even when sought appropriately, non-O157:H 7 SLTEC are rarely found in stools submitted for bacterial culture in North America compared to their frequency in the environment (8,52,59). Moreover, E. coli O157:H7 is the predominant precipitant of the hemolytic uremic syndrome (HUS), the most important complication of enteric infection with E. coli O157:H7. For example, E. coli O157:H7 was found in 96% of HUS patients if stool was obtained within the first six days of diarrhea (72). Even though non-O157:H7 SLTEC have caused some cases of HUS in several foreign series (10,11,12,35,40), these strains have never been reported to cause HUS in the United States. These data suggest that E. coli O157:H7 is the most important cattle-borne human pathogen threatening the food supply of this country today.
Cattle are the only reservoir of E. coli O157:H7 so far identified. Approximately 1 in 200 apparently healthy northwestern United States dairy and beef cattle carry E. coli O157:H7, and 8 to 16% of herds have at least one infected animal (25). Similar carriage rates have been detected nationwide (26). These are probably minimum carriage rates, because the technique used to culture E. coli O157:H7 is relatively insensitive.
A very low inoculum of E. coli O157:H7 can cause human disease. Person to person spread occurs rather easily in outbreaks and among sporadic cases (5,6,60). Microbiologic analysis of the contaminated hamburger from the 1993 Western United States outbreak demonstrated that only approximately 200 E. coli O157:H7 were present in each of the contaminated patties (46). It is probable that the inadequate cooking that was applied reduced this concentration by at least one log, suggesting that very few E. coli O157:H7, perhaps in the range of 1-10 bacteria, can cause clinically apparent infection.
Data suggest that the incidence of diseases caused by E. coli O157:H7 has increased in the United States, independent of ascertainment bias by diagnosing physicians (44,70). Additionally, an increasing rate of antibiotic resistance in Washington State human isolates of E. coli O157:H7 might portend an increased prevalence of this pathogen in animals administered antibiotics. For example, before 1988, non of 56 strains of E. coli O157:H7 were resistant to a wide variety of antibiotics tested, whereas after 1988, 7.4% of 176 strains were resistant to the same combination of antimicrobials (streptomycin, sulfamethoxazole, and tetracycline). It is probable that the selective pressure for the acquisition of antibiotic resistance in E. coli O157:H7 occurred in farm animals. This emerging resistance is of considerable concern because such strains might achieve a selective advantage over other coliform bacilli in cattle given antibiotics, thereby increasing the frequency with which food of bovine origin is contaminated with this pathogen.
Because of the ease with which E. coli O157:H7 can cause human disease, it is crucial to reduce this pathogen in, or eliminate if from, its ecological niche, namely the gastrointestinal tracts of healthy cattle.
The molecular mechanisms used by E. coli O157:H7 to adhere to epithelial cells and colonize animals are poorly characterized. However, the adhesive properties of E. coli O157:H7 have been noted by several investigators. Most North American strains of E. coli O157:H7 displayed D-mannose-resistant adherence patterns to HEp-2 or Henle 407 cells (57). Most strains adhere in the form of localized microcolonies, a phenotype strongly linked to diarrhea in epidemiological studies of enteropathogenic E. coli (EPEC) (13,16). A 60 MDa plasmid is present in all strains of E. coli O157:H7, and one group associated the expression of sparse D-mannose-resistant adhesion to Henle 407 cells to the presence of this plasmid (34). Plasmid-cured E. coli O157:H7 expressed no fimbriae and were nonadherent, and a 60 MDa plasmid from E. coli O157:H7 conferred weak adherence to non-adherent E. coli C600. However, other investigators have shown that plasmid-less E. coli O157:H7 were fimbriated, whereas laboratory E. coli strains were not (79). furthermore, plasmid-cured E. coli O157:H7 adheres to epithelial cells as well or better than its parent (22,33). Only one of five adherent strains of E. coli O157:H7 studied by Sherman et al. (66) was fimbriated, but this fimbriated strain also agglutinated erythrocytes. The agglutination was sensitive to D-mannose, suggesting that this adherence was due to type I fimbriae. Taken together, these data suggest that an identifiable fimbrial structure is not responsible for the adherence of most E. coli O157:H7 to Henle 407 cells.
Outer membranes of E. coli O157:H7 competitively inhibit adherence to HEp-2 cells, an inhibition which is not due to H7 flagellin or O157 lipopolysaccharide (65). Adherence of E. coli O157:H7 to HEp-2 cells was reduced, but not abolished, by antibody to a 94 kDa outer membrane protein (64). Antibodies to enterotoxigenic E. coli colonization factor antigens I and II do not detect surface structures on E. coli O157:H7 (78). E. coli O157:H7 do not have sequences homologous to the EPEC adherence factor plasmid or to the diffuse adherence adhesin (71).
Some investigators have suggested that the epithelial cell adhesion of E. coli O157:H7 is encoded by its eae gene (17). E. coli O157:H7 eae is related to inv, which encodes Yersinia invasin, which also functions as an adhesin, and EPEC eae, which encodes intimin. An eae deletion mutant of E. coli O157:H7 neither adhered to HEp-2 cells nor caused the attaching and effacing (AE) lesion in newborn pigs (17). When deletion mutants were complemented in trans by an intact eae gene, the strain could again cause the AE lesion, but still could not adhere in vitro. However, data from other groups suggest that the eae gene product is not an adhesin for E. coli O157:H7. First, despite sequence homology to inv in its bacterial localization and transmembrane domains, the receptor binding domain of E. coli O157:H7 eae is quite dissimilar (4,82). Second, an eae insertional mutant in E. coli O157:H7 retained the ability to adhere to HEp-2 cells in a quantitative adherence assay (41). Third, an eae gene product does not confer adherence on nonadherent laboratory strains of E. coli. (Jerse, A., et al., Proc. Natl. Acad. Sci. USA 87:7839-7843, 1990) Thus, a molecule other than the eae gene product in E. coli O157:H7 appears to be the primary adhesin of E. coli O157:H7 for bovine epithelial cells, enabling this human pathogen to colonize the bovine gastrointestinal tract.
Bacterial adhesins, when used as immunogens, prevent disease or colonization of mucosal surfaces by bacteria in many animals (1,18,21,29,30,36,49,50,55,61,68,81, which are hereby incorporated by reference). The reduction of E. coli O157:H7 at its bovine source would enhance the microbiologic safety of food derived from cattle, and lessen the environmental biohazard risk posed by the approximately 100,000 cattle detectably infected with E. coli O157:H7 at any one time in the United States. The availability of antibody for passive immunization would greatly mitigate the harm engendered by outbreaks of this infection.